The present invention relates generally to thin-walled spiral duct commonly employed in air conditioning and heating installations, and more particularly, to apparatus for forming round spiral ductwork to oval.
Round, thin-walled spiral seamed metal ducts made from strips of sheet metal are widely employed in heating and air conditioning installations, as well as in other air ducting installations. Such spiral duct is efficiently produced in desired lengths by Tubeformer machines, manufactured for example by Spiral-Helix, Inc. of Buffalo Grove, Ill., and described for example in Castricum U.S. Pat. Nos. 4,567,742; 4,706,481 and 4,711,110. Spiral duct has a number of advantages, in both ease of installation and performance.
There are however installations where limited space prevents the use of round spiral duct, and where, accordingly, flatter ductwork which is rectangular or oval in cross-section is employed. (Oval ductwork is also known in the art as "flat-oval," and the terms "oval" and "flat-oval" are employed interchangeably in the context of the invention.) As the Tubeformer machines produce round and not oval ductwork, a separate machine, known as an ovalizer, is ordinarily employed to form round ductwork to oval.
Very briefly, a typical ovalizer includes a pair of elongated and laterally spaced-apart duct forming members that are sized to fit within a ductwork section and to bear against opposing interior surfaces thereof. The duct forming members are forced apart in some manner to form the round ductwork to oval, slightly stretching the metal, perhaps by one percent, so that the formed oval shape is maintained. Extreme force is required, approximately 34,000 pounds per lineal foot, or approximately 340,000 pounds total force to form a section of ductwork ten feet in length.
One general type of ovalizer which has been proposed employs a plurality of pivoting mechanical linkage or toggle arms spaced longitudinally along the duct forming members and connected to an intermediate actuator rod driven in a longitudinal direction by an hydraulic cylinder. The duct forming members are forced apart as the actuator rod moves longitudinally with reference to the duct forming members. Longitudinal motion of the actuator rod is thus mechanically translated to motion at a right angles as the duct forming members move apart relative to each other.
An inherent beneficial result of this mechanical arrangement, subject to manufacturing tolerances, and subject to bending of the duct forming members, is that the duct forming members move apart in a parallel manner, which is assured by the geometry of the linkage mechanism and actuator rod. In other words, all points along each of the duct forming members inherently move together the same distance. Examples are disclosed in Jones U.S. Pat. No. 3,713,609 and Meserole U.S. Pat. No. 3,996,783. Related mechanisms, for slightly different purposes, are disclosed in Cunningham U.S. Pat. No. 3,747,394 and Haws U.S. Pat. No. 4,862,724.
Such arrangements using mechanical linkage or toggle arms and a longitudinally driven rod for right angle motion translation have several drawbacks. One drawback is that the available force is not constant over the range of motion, since the mechanical advantage increases as the duct forming members move farther apart. Thus, when smaller ductwork sizes are formed from round to oval, relatively less force is available for stretching the metal, compared to when larger ductwork sizes are formed from round to oval and the duct forming members correspondingly are farther apart. Another disadvantage is that the physical size of the linkage arms required to bear the force can limit the range of motion.
A related functional principle is employed in ovalizers manufactured by Spiral-Helix, Inc. such as the commercially available Spiral-Helix, Inc. models known as "The Helix Ovalizer 36/12" and "The Compact Helix Ovalizer 24/6." It is believed that those two machines employ an inclined plane mechanism wherein one of the duct forming members includes a series of inclined plane surfaces, in the form of hardened steel wedges. Corresponding inclined plane surfaces are provided on a actuator rod which is positioned between the two duct forming members and which is driven longitudinally by a hydraulic cylinder. Thus, as the actuator rod and its inclined planes move relative to the duct forming members, which are restrained against longitudinal movement, sliding motion of the inclined plane surfaces against one another forces the duct forming members apart. Again, the mechanism effects right angle motion translation. The stroke of the Spiral-Helix, Inc. ovalizer is six inches.
"The Compact Helix Ovalizer 24/6" and "The Helix Ovalizer 36/12" are equipped with sets of semi-cylindrical heads of different diameters corresponding to the minor axis of different sizes of oval ductwork after forming, as well as a set of spacers that can be selectively installed as required.
Despite the apparently simple principle of the ovalizers manufactured by Spiral-Helix, Inc., the practical implementation is difficult. Relatively high pressure is applied to wearing parts, which accordingly must be maintained. Moreover, as the duct forming members move farther apart, there is less contact area available between the corresponding inclined plane surfaces. Thus, as the duct forming members move apart to the point where the duct metal begins to stretch, less contact area is available, concentrating increasing pressure on decreasing areas of the inclined plane surfaces.
As a result, the list price of a full size "The Helix Ovalizer 36/12" with duct forming members twelve feet in length (known as "mandrels" in the terminology employed by Spiral-Helix, Inc.) is approximately $250,000.00.
One limitation of ovalizers employing a right angle motion translation mechanism is that design compromises must be made among factors such as the length of the duct forming members (and therefore the longest section of ductwork that can be formed), the stroke (the distance of the relative movement of the duct forming members between the fully retracted position and the position where they are farthest apart), the force and stroke of the hydraulic cylinder used to drive the actuator rod, and structural requirements of the actuator rod. The mechanical advantage of the motion translation mechanism (either pivoting linkage arms or inclined planes) determines the relationship of the stroke of the duct forming members to the stroke of the actuator rod. Increasing either the stroke of the duct forming members (greater strokes are desirable) or the length of the duct forming members in general requires a greater force from the hydraulic cylinder.
As an alternative to mechanical linkages for translating longitudinal motion of an actuator rod to uniform motion of duct forming members away from each other, it has also been proposed to force the duct forming members apart by means of directly-attached hydraulic cylinder assemblies, with no motion translation required. One example is disclosed in Goodwin U.S. Pat. No. 4,571,980. The disclosed Goodwin duct-shaping machine employs a pair of duct forming members, reinforced with "I" beams, that are effectively placed inside a ductwork section to be formed. An hydraulic cylinder assembly is connected at each end of the duct forming members to force them apart. The disclosed Goodwin machine in addition has a pair of side compactors which engage exterior surfaces of the ductwork section being formed so as to flatten the sides of the duct along a minor axis, while the interior duct forming members are moving apart along the major axis. A related apparatus for "arching" corrugated steel drainage pipes, related in that hydraulic cylinders are employed to directly force out pipe forming segments, is disclosed in Brinegar U.S. Pat. No. 4,914,939.
Notwithstanding the disclosure of Goodwin U.S. Pat. No. 4,571,980, there are significant practical difficulties in devising an ovalizer which employs direct actuation of the duct forming members by hydraulic cylinders, which perhaps explains in part why the right angle motion translation mechanism approach of the ovalizers manufactured by Spiral-Helix, Inc. are what, heretofore, have primarily been available in the marketplace.
A significant factor is the relatively high force involved to form round ductwork to oval (at least in the absence of the minor axis compaction assemblies of Goodwin), and additionally to stretch the sheet metal approximately one percent, by slightly exceeding the yield strength of the duct metal, so that the oval shape is retained. The force required ranges from approximately 20,000 pounds per lineal foot along the lengths of the duct forming members in the case of 26 gage ductwork to 34,000 pounds per lineal foot along the lengths of the duct forming members of the case of spiral duct made of 16 gage sheet metal. A practical ovalizer has duct forming members that are ten feet in length, requiring a total force of approximately 340,000 pounds. Providing a structural beam to sustain such forces without undue bending if driven from the ends as disclosed in Goodwin U.S. Pat. No. 4,571,980 would be quite difficult.
The pipe arching apparatus of Brinegar U.S. Pat. No. 4,914,939 employs a plurality of cylinders, but still only two per individual forming segment. The requirements of a pipe arching apparatus, as disclosed in Brinegar U.S. Pat. No. 4,914,939, and the requirements of a spiral duct ovalizer differ significantly.